The present invention relates to a train of equipment to continuously manufacture steel wire and, more specifically, to a train of equipment to continuously manufacture steel wire of carbon steel for machine structure use or alloy steel having an excellent cold working property.
The present invention relates to temperature controlling apparatuses for hot rolled steel wire and, in more detail, to temperature controlling apparatuses which are so designed that desired treatment patterns can be chosen, in one line, from among various kinds of cooling and heat retention treatments and heat treatments of wire in coils in accordance with the material quality and the final use of hot rolled steel wire.
A common process to manufacture steel wire comprises the steps of; heating a billet to a prescribed temperature in a reheating furnace, hot rolling the billet into wire of an intended size, winding the wire into continuous rings, cooling and appropriately heat-treating the wire, packing the wire into a bundled coil, and banding the coil. The bundled coil may undergo treatments such as annealing as required, before being shipped to a secondary work process. Various proposals have been made regarding the hot rolling and heat treatment steps of the steel wire manufacturing.
Looking at hot rolling of steel wire from the apparatus viewpoint, for example, a block mill, developed as a finish rolling mill for steel wire, has advantages especially of high speed rolling, compact equipment design and fewer surface defects. A block mill, in which 8 to 10 roll stands are closely arranged in tandem in one frame, can roll a material without twisting it and, for this reason, it has been introduced in many rolling lines recently.
Looking at the hot rolling of steel wire from the viewpoint of material property and structure, it is possible to refine a xcex3 structure by employing a method to hot-roll at as low a temperature as possible, for example not exceeding 800xc2x0 C., and make the rolling finishing temperature lower than that in normal rolling practices (such a rolling method being hereunder referred to as controlled rolling). A technology has been known to soften the material of steel wire by dividing and granulating a laminated cementite of a pearlite structure through a combination of the above rolling method with slow cooling in the downstream process steps. However, since the rolling finishing temperature is usually 900xc2x0 C. or higher in the normal practice of steel wire rolling, the refinement of xcex3 structure cannot be achieved, and it is necessary to anneal steel wire off-line to soften the wire material.
Japanese Patent No. 2857279 discusses a conventional example of using a rolling mill resembling the one employed in the present invention. FIGS. 1 and 2 of the patent show an equipment configuration where a 4-stand post-finishing block mill is provided after an 8-stand finishing block mill to realize free-size rolling and precision rolling. In addition, the patent also proposes to provide a cooling apparatus at the entry side of the post-finishing block mill.
In the meantime, various methods have been proposed such as the one to wind and spread hot rolled steel wire into non-concentric rings and subject it to a direct heat treatment in the process of packing it into a bundled coil. The Stelmore method is an example of such proposals. Among these proposals, a means disclosed in Japanese Examined Utility Model Publication No. H4-37898 to construct a winder (laying cone), a transportation route for the wire in continuous rings and a heat retention furnace covered with a closed heat retention cover, and a technology disclosed in Japanese Examined Patent Publication No. H7-98977 to provide a line for normal heat treatment and another line for slow cooling, in a manner to allow switching, and to feed steel wire to a conveyer of a selected succeeding process, can be counted as conventional examples to treat steel wire in the form of bundled coils for the purpose of slow cooling after the winding.
With a finishing mill such as the block mill mentioned above, however, the total area reduction rate through 8 stands is as high as about 85%, and controlled rolling is practically impossible with hard materials generating large amounts of heat during working and used mainly for machine structure, such as carbon steels with 0.4% or more of carbon, alloy steels, spring steels and bearing steels. Further, in the Japanese Patent No. 2857279 mentioned above, a 4-stand block mill is installed as a finish rolling mill and a cooling apparatus is provided at the entry side of the mill. This arrangement, however, aims at suppression of abnormal growth of crystal grains and not at on-line manufacturing of soft steel wire having an excellent cold working property through a combination of grain size refinement by controlled rolling with a cooling means in a succeeding process, which is the feature of the present invention.
In the above-mentioned Japanese Examined Utility Model Publication No. H4-37898, a unique structure is employed where a winder is covered with a closed cover and, for this reason, there is a problem in terms of equipment costs, since a special apparatus is required exclusively from winding to slow cooling and thus most of existing wire manufacturing facilities cannot be used. Further, according to the above-mentioned Japanese Examined Patent Publication No. H7-98977, since pot type furnaces are employed for slow cooling of bundled coils, there are problems of difficulty in individually controlling the temperature, low productivity and the process not being suitable for continuous operation. In addition, since slow cooling starts in these conventional slow cooling lines from a comparatively high temperature of 850xc2x0 C. or higher, there is a drawback that the line length inevitably tends to be long.
Besides the above, various methods of controlled cooling of steel wire are practiced during transfer on a conveyer after hot rolling, winding into rings using a winder having a laying head and spreading onto the conveyer. These methods include cooling by air blast, leaving to cool naturally (these two methods being hereunder simply referred to as, respectively, blast cooling and natural cooling), rapid-cooling actively with water or, otherwise, cooling slowly or retaining heat by covering a transfer line with a heat retention cover.
For example, Japanese Examined Patent Publication No. S60-55572 discloses a technology whereby hot rolled steel wire laid on a conveyer in rings is cooled with an air blast or a water spray and then, after being packed into bundled coils on pallets, is charged by a branching conveyer into an annealing furnace for a continuous heat treatment. Cooling with a water spray has a shortcoming in that it is incapable of cooling evenly and the wire material becomes inhomogeneous. What is more, this technology inevitably requires very large equipment occupying a huge area, resulting in a big disadvantage in the plant space requirement. The same publication discloses also a water cooling method used after forming the wire into bundled coils, but this method results in a highly inhomogeneous cooling.
Also, Japanese Unexamined Patent Publication No. H6-336620 discloses a technology whereby hot rolled steel wire laid on a conveyer in rings is rapid-cooled by directly submerging it into a cooling tank, then, after being packed into bundled coils, heat-treated (quenched and tempered) in a tempering furnace. This technology, however, employs a method to heat the bundled coils suspended on a hook conveyer. Since a maximum furnace atmosphere temperature in this method of transportation cannot surpass 650xc2x0 C. or so as the bundled coils deform at a temperature exceeding 650xc2x0 C., the method has a problem in that it is inapplicable to a quick heat treatment at high temperatures.
Further, Japanese Unexamined Patent Publication No. H8-193222 proposes apparatuses to selectively supply hot rolled wire to different lines for different kinds of heat treatment. According to the technology disclosed therein, in transferring steel wire wound into rings on a conveyer, heat-treating it and packing it into bundled coils on the conveyer, the wire is first packed into bundled coils after winding and is then transferred to a separate line for cooling in an immersion type cooling apparatus or, otherwise, bundled coils of wire are covered individually with heat-insulating hoods and gathered in a covered pit for heating, and processing lines for these and other heat treatments are arranged so that each of them may be selected as required. In this technology, however, the various heat treatment lines are arranged on a same plane and, for this reason, operability is poor due to an entangled layout of the lines and the disadvantage, in terms of required space, is great. Moreover, steel wire in coil cannot be cooled evenly because, when a bundled coil is immersed into water, water does not infiltrate into the interior.
As can be seen from the above, among past technologies related to rolling and heat treatment of steel wire, no example where controlled rolling and slow cooling are considered in combination with each other can be found.
From the above, therefore, materialization of an economical train of equipment to continuously manufacture steel wire of carbon steel for machine structure use or alloy steel having an excellent cold working property, wherein a controlled rolling means using a block mill and slow cooling means are rationally combined in one continuous line and high level operation both of the rolling and slow cooling can be achieved, and the train of equipment can be easily incorporated into an existing line, is strongly desired.
An object of the present invention, which was completed in view of the above situation, is to provide a train of equipment, to manufacture steel wire, capable of easily realizing controlled rolling (low temperature rolling), which has been conventionally regarded as being difficult with a block mill, and efficiently performing the controlled rolling and slow cooling through an effective in-line combination of a controlled rolling apparatus with slow cooling apparatuses. Another object of the present invention is to provide a train of equipment, to manufacture steel wire, capable of manufacturing every size of steel wire while eliminating annealing processes, which have been considered indispensable in secondary working stages which requires as short a line as possible for slow cooling.
A further object of the present invention is to remarkably expand the degree of freedom in in-line treatment of hot rolled steel wire by continuously combining a controlled cooling section, for steel wire in rings, covering a wide range of cooling methods from water cooling to slow cooling with a heat treatment section for the wire in coils as well as to make the space required for equipment as small as possible by vertically and rationally arranging a controlled cooling zone, including an immersion cooling means, and by providing a common conditioning and banding line for coils paid off from each of the treatment lines.
A train of equipment to continuously manufacture steel wire according to the present invention for achieving the above objects is characterized by sequentially connecting a hot rolling mill, to roll billets of carbon steel for machine structure use or alloy steel to a desired diameter, a winder to wind and form the rolled steel wire into rings, bundling apparatuses to pack the wire in rings into bundled coils and an in-line heat treatment furnace to slow-cool the wire packed into bundled coils, and by using a block mill having at most 4 roll stands as a final finishing mill of said hot rolling mill. In this configuration, the restriction of the area reduction rate of the final finishing block mill to a range of 25 to 60% and the design of the mill to have at most 4 roll stands prevent the generation of excessive working heat and make the envisaged controlled rolling possible.
The train of equipment to continuously manufacture steel wire according to the present invention is characterized, besides the above, in that an in-line heat treatment furnace has a capacity to accommodate xc2xc to all of the number of bundled coils of wire rolled in 1 hr. at the maximum rolling capability. Even with this capacity, the heat treatment furnace can cool the wire at a very mild cooling rate of 0.1xc2x0 C./sec., or slower, and slow enough to cause division and granulation of laminated cementite of pearlite structure and thus the slow cooling line does not need to be very long.
Further, the train of equipment, to continuously manufacture steel wire, according to the present invention is characterized, besides the above, by having, between a winder and bundling apparatuses, a controlled cooling and transporting apparatus equipped with heat retention covers to transfer steel wire, formed into continuous non-concentric rings, while holding it without lowering the temperature. The train of equipment is characterized also by winding steel wire using a winder at a temperature not below the Ar1 transformation point, transporting the wound wire without lowering the temperature below the Ar1 transformation point in a controlled cooling and transporting apparatus having heat retention covers and feeding the wire to bundling apparatuses and a slow cooling line. With the holding and transportation apparatus, it is possible to feed the steel wire in rings always in a stable condition to the slow cooling line without involving a special winding apparatus such as a one covered with a closed cover. The apparatus also makes it possible to apply the present invention to an existing manufacturing line easily and economically even when there is a restriction in the layout such that the distance between its winder and bundling apparatus is too long. In addition, the apparatus also has a function to homogenize the temperature of steel wire as well as to control the wire temperature to the commencement temperature of the succeeding slow cooling process by properly controlling the atmosphere temperature inside the apparatus when transferring the wire inside the heat retention covers.
Additionally, the train of equipment to continuously manufacture steel wire according to the present invention is characterized, further to the above, by having, at the entry side of a final finishing mill, a water cooling and recuperation zone having a length equal to or longer than {fraction (1/10)} of the distance traveled by the steel wire in 1 sec. at the maximum rolling-speed. The water cooling and recuperation zone-makes controlled rolling possible by supplying the final finishing block mill with the desired steel wire without causing material deterioration.
The train of equipment to continuously manufacture steel wire according to the present invention is further characterized by packing steel wire, bundled in bundling apparatuses, into tight coils by the use of stems. inserted into the inner space of the coils and transporting the coils to a succeeding in-line heat treatment furnace. Feeding the steel wire coils tightly packed, rather than loosely, to the heat treatment furnace prevents a slow cooling line from becoming too lengthy, makes the mild cooling effective and secures stability during the transportation.
The train of equipment to continuously manufacture steel wire according to the present invention is further characterized, in terms of specific equipment configuration: by having;
No. 1 controlled cooling zone easily and selectively convertible into any of a water cooling line, a blast cooling and natural cooling line and a slow cooling and heat retention line at the exit side of the winder to wind the hot-rolled steel wire into rings using a laying head,
No. 2 controlled cooling zone easily and selectively convertible into any of a blast cooling and natural cooling line and a slow cooling and heat retention line, succeeding said No. 1 controlled cooling zone, a transfer means to transfer the steel wire in rings between the Nos. 1 and 2 controlled cooling zones and No. 1 bundling apparatus to pack the steel wire rings into bundled coils in a manner that each of them can be alternatively placed at a boundary position between Nos. 1 and 2 controlled cooling zones,
No. 2 bundling apparatus at the rear of said No. 2 controlled cooling zone, and
a heat treatment means to slow-cool or heat the bundled coils of steel wire packed by said No. 1 bundling means connected to the position where said No. 1 bundling means is installed: and
by selectively using the controlled cooling zones and/or the heat treatment means in accordance with the temperature pattern required for obtaining desired material properties of the steel wire.
The above configuration, in which the controlled cooling section of the wire in rings is divided into Nos. 1 and 2 controlled cooling zones, No. 1 controlled cooling zone has the functions of 3 selectable lines, namely a water cooling line, a blast and natural cooling line and a slow cooling and heat retention line, the No. 2 controlled cooling zone has the functions of 2 selectable lines, namely a blast and natural cooling line and a slow cooling and heat retention line, both the retractable transfer means and the No. 1 bundling means to pack the wire rings into bundled coils are provided at the boundary between Nos. 1 and 2 controlled cooling zones, and No. 1 bundling means is connected with the heat treatment furnace, makes it possible to carry out on-line, in addition to ordinary blast cooling and natural cooling of the steel wire in rings, treatments conventionally carried out off-line such as heating of packed wire coils after a water cooling (immersion cooling) of the wire in rings, holding bundled coils after blast cooling or natural cooling of the wire in rings, and slow cooling of bundled coils after slow cooling and heat retention of the wire in rings.
The train of equipment for continuously manufacturing steel wire according to the present invention is further characterized in that a temperature controlling apparatus for steel wire after winding has, as a water cooling means of No. 1 controlled cooling zone, a cooling tank to directly immerse steel wire and a capability to use either cold water or hot water alternatively. It is preferable to install the immersion cooling tank beneath the transportation line for the blast and natural cooling and slow cooling and heat retention to make the equipment arrangement space-efficient.
Further, above-mentioned temperature controlling apparatuses are characterized by employing heat retention covers or heat retention covers with heat sources as slow cooling means in Nos. 1 and 2 controlled cooling zones. The heat retention covers secure a very low cooling rate of the steel wire in rings passing through them and are useful for efficient utilization of rolling heat. The steel wire may be actively heated by the heat sources when required. The temperature controlling apparatuses are further characterized by their continuous arrangement at the rear of a steel wire rolling line having, as a finishing rolling mill, a high rigidity block mill with a mill rigidity of at least 40 ton/mm or more. The manufacturing equipment of the above arrangement rationally combines the controlled rolling apparatuses with the cooling apparatuses to efficiently manufacture steel wire having an excellent secondary working property.
Additionally, a heat treatment means for slow cooling or heating of the steel wire in bundled coils may be of a tunnel type to receive and transport the coils or of a pot type to cover each of them. Either of the types may be selected in consideration of factors such as ease of operation, relationship with other facilities and cost efficiency.
In addition to the above, the train of equipment to continuously manufacture steel wire according to the present invention is characterized by transporting and supplying steel wire coils, coming from No. 2 bundling means and a heat treatment means, to a common conditioning and banding means. This arrangement realizes a rational and compact layout of the whole manufacturing line equipment, resulting in great advantages in the plant space and work efficiency.